Many human activities require a near-instant decision about how to react to a visual cue. Although traditional models emphasized a serial progression from perception to decision and only subsequently to action, this idea has been regularly challenged within the human behavioral literature. Furthermore, neuroscientific investigations have revealed that time-varying neural activity in the primary motor cortex (M1) can actually reflect cognitive processes in a seemingly continuous manner.
In the past decade, considerable evidence from trained monkeys has suggested a role for neural computations of motor origin in the decision process.
A handful of studies have now attempted to provide neurometric evidence in humans to support the role of the sensorimotor (and particularly the motor) cortex in the decision-making process….By requiring lateralized responses to indicate “present”/“absent” decisions, they showed a clear correlation between motoric beta and the process of evidence accumulation.
Indeed, previous modeling work in humans has demonstrated marked differences in the neural processes underlying reward-based decisions as compared with state/stimulus-based decisions…perceptually more taxing decision resulted in longer motoric activity relating specifically to the selected action, despite equal motor requirements…(but) when MEP signals were locked to stimulus onset, gender discrimination difficulty had no significant effect on the time of MEP divergence (which provides a sensitive measure of when one response starts to be selected…. This finding challenges serial models of decision making, as under these accounts harder gender categorizations would be expected to delay the accumulation of evidence toward one of the two response alternatives and thus delay action selection.
Instead, the response-locked analysis showed that response-specific processing in motor regions successfully predicted the increased RT produced by adjusting the difficulty of the perceptual discrimination…
This study therefore provides neurophysiological evidence of a continuous flow of information regarding the decision to the motor system, and that this evidence accumulation occurs at the intrahemispheric level. It suggests that motor activity is not instigated only after the preceding cognitive processes have terminated. Rather, the motor hot spot continuously reflects, and possibly even helps determine, the dynamics of the decision process…Given that the link between flexion and extension is inhibitory in nature, demonstrating inhibition in the present design might have provided stronger evidence for the role of inhibitory processes in selection. However, MEPs ride atop general (e.g., spinal-level) modulations in excitability, so a drop in amplitude does not necessarily imply selective inhibition of a motor plan…Their findings are consistent with ours, showing a continuous update projected downstream to the muscles as the evidence for a particular response gradually accumulates. Thus, converging data from different levels in the motor system suggest that the perceptual brain continuously conveys decision information to motor cortex…
The current findings are also consistent with evidence against serial/sequential models in the interhemispheric domain in man..corticospinal excitability can reliably predict the outcome of a decision prior to completion of the decision process…the point of separation between the response-locked MEP signals of the chosen and unchosen responses occurs earlier in hard decisions as compared with easy decisions…data in the cognitive (value-laden) decision domain closely match our findings in the perceptual decision domain…Thus, it can tentatively be concluded that both intra- and interhemispheric response selection is represented continuously in motor cortex.”
Source: Psychophysiology, 00 (2016), 00–00. Wiley Periodicals, Inc. Printed in the USA. Copyright VC 2016 Society for Psychophysiological Research DOI: 10.1111/psyp.12737